So I did the tappet clearance on my car after doing a head gasket change, and evidently I got it wrong. The car starts (with more difficulty than is usual) then seems to run ok whilst its cold, but once it heats up it loses all power and wont run.

i set the gap as per the workshop manual to somewhere between 12 and 13 thou but maybe I didnt do it right. Question is, with the above symptoms (almost complete power loss once car is hot) have I made the gap too small or too big?

if it's the tappets wrong, then too small - the gap closes when everything heats up so holding the valves open a little bit...

but go over everything else as well, if you did the tappets as per the manual with the rule of 9's etc. then they should be fine - it'll be something silly like the choke stuck on

You have made them too close.
When hot, it can hold the exhaust valves off their seats.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

did you run the engine, retension the head then do the tappets ?

what is the correct hot clearance?

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A good friend will come and bail you out of jail...but a true friend will be sitting next to you saying "Damn...that was fun!"

For a stock Mini back in the 60s, most mechanics used to run .012" cold or .011 hot.
These days with sportier cams, you don't set them so close. Even a stock Cooper S used in competition needs .015" clearance.

Kent cams and GR's RE13 and many others use .016" with stock rockers, add .003" for 1.5s.
My RE282 is .022" inlet and .024" exhaust (with 1.5 rockers).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.